child_process: Add gid/uid flags to spawn config

This is mostly working, but not completely ideal for two reasons.

1. Rather than emitting an error on the ChildProcess object when the
setgid/setuid fails, it is simply printing the error to stderr and
exiting.  The same happens with the cwd, so that's not completely
terrible.

2. I don't have a good test for this.  It fails with an EPERM if you try
to change the uid or gid as a non-root user.